[Walt Fricke]

By and large, brushes are replaced when the VR craps out. Or you just get a new VR, with its new brushed attached (they all come that way).

If you have a lathe, you might be able to turn the commutator (that brass segmented thing the brushes contact if I have my terminology right). I don't know that using sand paper is going to clean a worn one up suitably.

Which is why going to a specialty shop is the path of least resistance for most of us, even frugal DIY guys like me (though I don't have a lathe anyway). They are all set up for everything, can test output to start with, and then after they are done.